Electro-mechanical printing apparatus

An electro-mechanical printing apparatus including in combination

• a platen,
• means for advancing electro-sensitive paper over the platen,
• a print head cooperating with the platen, the print head having a number of selectively energisable needles arranged in a row in the print head,
• an electric motor,
• drive transmission means coupling the electric motor to the print head to effect translational movement of the latter parallel to the platen in a direction transverse the row of needles,
• a timing disc coupled to the drive transmission to be rotated thereby, the disc being subdivided into equi-angular sectors with adjacent sectors being of different light-transmitting or reflecting characteristics,
• photoelectric sensing means arranged to cooperate with the timing disc to produce periodic signals upon rotation of the disc, the photoelectric sensing means including two photocells displaced relative to each other around the timing disc so that the signals produced by the two photocells upon rotation of the disc are in phase quadrature relative to each other, and
• clock pulse generating means responsive to the periodic signals produced by the two photocells to provide a clock pulse each time a sector boundary passes either photocell, said clock pulses being utilised to control print needle energisation and having a phase of generation relative to said periodic signals dependent on the direction of timing disc rotation and therefore on the direction of print head movement.